It is necessary to export __pthread_cond_destroy from libc because
the C11 condition variable needs it and is still left in libpthread.
This is part of the libpthread removal project:
<https://sourceware.org/ml/libc-alpha/2019-10/msg00080.html>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>